1. Field of the Invention
The present invention relates to a charged particle beam generating apparatus adapted to be incorporated in an electron beam exposure apparatus, an electron microscope, an ion beam exposure apparatus or the like, and particularly best suited for use in an electron gun of the field emission type or the thermal field emission type. More specifically, the invention relates to such a charged particle beam generating apparatus which can increase an effective beam emission angle without lowering the brightness, and can enhance the efficiency of use of the beam current.
2. Description of the Prior Art
In an electron beam exposure apparatus for describing or extracting fine patterns in the fabrication of an LSI, it has been required to improve the brightness of an electron gun in order to improve a description resolution and a description speed. In an electron microscope, it has also been required to improve the brightness of an electron gun in order to improve an observation resolution.
The brightness B of an electron gun is expressed in the following: EQU B =I'/ r.sup.2 ( 1) EQU r =[r.sub.o.sup.2 +(0.25 C.sub.ss.sup.3).sup.2 +(0.5C.sub.c s .DELTA.V/Va).sup.2 +(0.6.lambda./s).sup.2 ].sup.1/2 ( 2)
where I' represents an angular intensity (electric current per unit solid angle) of the beam emitted from an emitter, r represents the radius of a beam emitting source obtained when taking aberrations into consideration, s represents an emission angle (beam half angle), r.sub.o represents the radius of the beam emitting source obtained when disregarding the aberrations, Cs represents a spherical aberration coefficient (referred to the object side) of the electron gun, Cc represents a chromatic aberration coefficient (referred to the object side) of the electron gun, .DELTA.V represents a beam energy spread, Va represents a beam accelerating voltage, and .lambda. represents a wavelength of the electrons at the accelerating voltage Va.
From the formulas (1) and (2), it will be appreciated that in order to enhance the brightness B, it is necessary (1) to reduce the radius r.sub.o of the beam emitting source obtained when the aberrations are disregarded and (2) to reduce the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc. For this reason, an electron gun, incorporating an emitter of the field emission type having a pointed distal end, an emitter of the thermal field emission type and a magnetic lens having small aberration coefficients, has now begun to be used as an electron beam generating apparatus.
As a first example of conventional charged particle beam generating apparatuses, there is shown in FIG. 1 "a high current efficiency field emission gun system incorporating a preaccelerator magnetic lens" of M. Troyon described in Optik 57, No. 3 (1980) p. 401. The electron gun is accomodated within a column 1 having an evacuation port 2 through which the interior of the column 1 can be evacuated to a high vacuum by a vacuum system (not shown). The electron gun comprises an emitter 3 made of a tungsten tip having a pointed distal end, a magnetic lens 4 composed of a first anode 5, a coil 6 and a pole piece 7, and a second anode 8. In this conventional structure, the inner surface of the column 1 is covered by a magnetic shield 9.
When a required voltage -Va (negative) is applied to the emitter 3 whereas a voltage -Va +Vo (hereinafter referred to as "extraction voltage Vo") is applied to the first anode 5 and the pole piece 7, an electron beam 10 is field-emitted from the distal end of the emitter 3. The second anode 8 is grounded, and the potential difference Va between the second anode 8 and the emitter 3 serves as a beam accelerating voltage. The first anode 5 and the pole piece 7 are both made of a magnetic material, and when electric current NI is caused to flow through the coil 6, a focusing magnetic field is created between the first anode 5 and the pole piece 7. The electron beam 10 is focused or converged by this focusing magnetic field on a point intermediate the pole piece 7 and the second anode 8. When the accelerating voltage Va, the extraction voltage Vo and the coil current NI are 90 kV, 1.85 kV and 380 AT, respectively, the spherical aberration coefficient Cs of this conventional electron gun is 2 mm, and its chromatic aberration coefficient Cc is 1.3 mm. These values are almost the lower limits of the aberration coefficients of field-emission type electron guns now available.
The positional relation between the pole piece and the emitter in the above first example of the field-emission type electron gun incorporating the magnetic lens is diagrammatically shown in FIG. 2 together with a distribution of the magnetic flux density on the optical axis. Reference numeral 11 denotes a principal plane of the lens defined by the pole piece 7 and the coil 6, and a curve 13 represents the magnetic flux density on the optical axis 12. In this conventional electron gun, the inner diameter of the pole piece 7 is uniform from its upper to its lower end, and the emitter 3 is disposed at a level above a space or gap between the opposed end surfaces 7A and 7B of the pole piece 7, that is, above the upper end surface 7A. In such a conventional electron gun, the magnetic flux density is the maximum at the center of the gap, and the distribution of the magnetic flux density is symmetrical. In such a construction, the principal plane 11 of the lens is disposed generally at the center of the gap, and the electron beam 10 is focused as shown in FIG. 2.
Generally, in an electron gun, the spherical aberration coefficient referred to the object side becomes smaller as the distance a between the distal end of the emitter and the lens principal plane becomes smaller. In the electron gun of FIG. 2, the spherical aberration coefficient Cs becomes smaller as the distance a becomes smaller. However, in the electron gun of FIG. 2, the emitter 3 is disposed at a level above the end surface 7A of the pole piece 7, and therefore the distance a could only be reduced to about a half of the length b of the gap at best. As a result, the reduction of the spherical aberration coefficient has been limited.
Next, as a second example of conventional charged particle beam generating apparatuses of the field emission type incorporating a magnetic lens, there is shown in FIG. 3 a field-emission type electron gun disclosed in Japanese Patent Application Laid-Open No. 84568/76. FIG. 4 shows a path of travel of an electron beam in this conventional electron gun. Those parts of FIGS. 3 and 4 corresponding respectively to those of FIGS. 1 and 2 are designated by identical and like reference numerals. In FIG. 3, reference numerals 7a and 7b denote pole pieces, and reference numeral 14a denotes a extracting electrode, and reference numeral 14b denotes an anode. The extracting electrode 14a and the anode 14b constitute an electrostatic lens. In FIG. 4, reference numeral 11a denotes a principal plane of a magnetic lens. In the case where an emitter 3 is disposed at a level above the peak of a magnetic flux density distribution 13, the principal plane 11a is formed generally at the peak of the magnetic flux density distribution 13. Reference numeral 11b denotes a principal plane of the electrostatic lens. Reference numeral 11c denotes a lens principal plane synthesized by the magnetic lens and the electrostatic lens. The electron beam 10 emitted from the emitter 3 in a radiating manner is caused by the focusing action of the magnetic lens to advance parallel with the optical axis 12, and is directed to an aperture of the extracting electrode 14a. The electron beam 10 passed through the extracting electrode 14a is focused by the focusing action of the electrostatic lens on a focusing point 10a. In this conventional electron gun, the magnetic field in the vicinity of the emitter 3 controls the radially-spreading electron beam, so that most part of the emission current can be directed to the electrostatic lens.
As shown in FIG. 4, in the second example of the conventional charged particle beam generating apparatus, the magnetic lens performs the function of making the radially-emitting electron beam 10 parallel to the optical axis 12, and the electrostatic lens performs the function of focusing the electron beam 10. Therefore, the lens principal plane 11c synthesized by the magnetic lens and the electrostatic lens is disposed generally midway between the principal plane 11a of the magnetic lens and the principal plane 11b of the electrostatic lens. On the other hand, the emitter 3 is disposed at a level above the peak of the magnetic flux density distribution 13 (that is, above the principal plane 11a). Therefore, the distance a between the emitter 3 and the principal plane 11c can only be reduced to about a half of the distance between the principal plane 11a and the principal plane 11b at best. As mentioned above, generally, the spherical aberration coefficient referred to the object side becomes smaller as the distance a between the emitter 3 and the lens principal plane 11c becomes smaller. However, in this second example of the conventional charged particle beam generating apparatus, the function of focusing the beam is performed by the combination of the magnetic lens and the electrostatic lens (this function is performed mainly by the electrostatic lens). Also, the emitter 3 is disposed at the foot of the magnetic flux density distribution disposed above its peak. Therefore the distance a between the emitter 3 and the lens principal plane 11c can not be reduced, which results in a problem that the spherical aberration can not be reduced as is the case with the above first example of the prior art.
Further, as a third example of conventional charged particle beam generating apparatuses, there is shown in FIG. 5 an electron beam apparatus of Kenneth C. A. Smith disclosed in U. S. Pat. No. 4,315,152. FIG. 6 shows a path of travel of an electron beam in this conventional structure. Those parts of FIGS. 5 and 6 corresponding respectively to those of FIGS. 1 to 4 are designated by identical and like reference numerals. In this conventional electron beam apparatus, an electrostatic lens, constituted by a extracting electrode 14a and two anodes 14b, is provided below and adjacent to an emitter 3. Provided below this electrostatic lens is a magnetic lens having a coil 6 and a pole piece 7 of the mono-pole type. In this construction, a principal plane 11 of the magnetic lens is disposed generally at a level of the upper end surface of the pole piece 7 of the mono-pole type, and the upper portion of the magnetic flux density distribution above its peak has a gently slanting configuration, and its foot end is disposed slightly above the emitter 3. Thus, in this conventional electron beam apparatus, the emitter is disposed at the foot portion of the gently slanting portion of the magnetic flux density distribution disposed above its peak. This results from the fact that the extracting electrode 14a (electrostatic lens) is disposed between the emitter 3 and the pole piece 7, and with this construction, the distance a between the emitter 3 and the lens principal plane 11 is long.
In the above-mentioned charged particle beam generating apparatuses, when the emitter 3 is made of a zirconium/tungsten thermal field-emission type tip, the following problems are encountered. Namely as described in "Emission characteristics of the ZrO/W thermal field electron source" (D. W. Tuggle and L. W. Swanson) in Journal of Vacuum Science And Technology, B3, No. 1 (1985), page 220, good characteristics of electron emission, in which the angular intensity is generally uniform, can be obtained at an emission angle s of not more than -35 mrad. However, in the conventional charged particle beam generating apparatuses, the spherical aberration coefficient Cs is large, and therefore when the electron gun is used at regions where the emission angle s is large, the brightness B of the electron gun has been greatly reduced. For example, in the first example of the conventional charged particle beam generating apparatus of FIG. 1, when a zirconium/tungsten thermal-field-emission type tip is used, the values determined by the formulas (1) and (2) are reduced to one-fifth (1,/5) where the electron beam accelerating voltage Va is 30 kV, the beam energy spread .DELTA.V is 0.8 eV, the angular intensity I' is 0.5 mA/sr, the radius r.sub.o of the beam emitting source obtained when disregarding the aberrations is 0.01 .mu.m, the spherical aberration coefficient Cs is 2 mm, and the chromatic aberration coefficient Cc is 1.3 mm. Therefore, in the aforesaid conventional charged particle beam generating apparatuses, the beam emission angle s is limited to not more than 15 mrad, which results in a problem that the beam current usable by a zirconium/tungsten thermal-field-emission type tip can not be fully utilized.